First, you should look closely to see if this meets the appropriate legal standards. Companies generally hold themselves to higher standards than the law requires, but if push comes to shove (as it well might in a situation like this) you can only make them follow the law.
Second, make sure this is really worth fighting. Even if you are in the right, the best move may be not to play. Consider updating your resume and looking for another job.
Third, document everything. Keep track of dates, people, witnesses, and details. Then file a formal complaint.
Fourth, file a formal complaint with HR. Make a good faith effort to get a proper resolution from HR and, again, document everything.
Fifth, if your complaint is mishandled, decide if you really, really want to fight this battle. If you do then you should contact a lawyer(BTW contacting one immediately is a good idea as well) to get their advice on how to proceed. You are walking into a legal minefield here and you need to be careful with your next actions.
If your boss has had any sexual harassment training for managers, he knows that this is the worst situation a company can possibly find itself. The penalties for harassing an employee into resignation are among the most severe. The company will, unless it's incredibly dumb, involve lawyers at this point.
Again, do you really want to do this? Are your coworkers willing to stand with you?
